We aim to understand how telomeres solve the end-protection problem. Specifically, this proposal describes experiments to gain insight Into the mechanism by which the telomeric protein complex, shelterin, represses the two main DNA repair reactions, NHEJ and HDR at chromosome ends. Our aim Is to delineate the nature of the repair reactions at telomeres, their outcomes, what factors are involved, how these pathways are regulated, and how telomeres prevent inappropriate pnscessing by repair pathways. Telomere dysfunction is thought to be main source of genome instability in human cancer, has been implicated in aging, and is at the core of several human diseases that are caused by loss of telomeric DNA (e.g. dyskeratosis congenita, anaplastic anemia). When telomeres become dysfunctional, inappropriate repair at chromosome ends is a major determinant of the pathological outcomes. Detailed understanding of the repair pathways at dysfunctional telomeres is therefore expected to impact the treatment and prevention human disease. We will use genetic, molecular genetic, and cell biological techniques to address the mechanism by which telomeres repress inappropriate repair and define the pathological repair reactions at dysfunctional telomeres. The experimental approaches will involve the tools for studying mammalian telomeres that we have developed under the auspices of this grant over the past 16 years. Specifically, we will use mouse embryo fibroblast cell lines from which shelterin components can be deleted with the Cre recombinase. By changing the genetic background of these cells, using compound genotypes, we can monitor the repair at dysfuncUonanl telomere, delineate the genes involved In the reactions, and define how shelterin represses these reactions at functional telomeres.